1. Field of the Invention
The present invention relates to a plasma monitoring method applicable to a semiconductor manufacturing processes (steps) and all the other manufacturing processes using plasma and a plasma monitoring system therefor.
2. Description of the Related Art
There is a conventional technique related to a plasma monitoring method and a plasma monitoring system for monitoring a processing on a wafer disposed in a plasma processing apparatus as disclosed in, for example, Japanese Patent Application Laid-Open (JP-A) Nos. 2003-282546 and 2005-236199.
FIG. 7 is a schematic configuration diagram showing a conventional plasma monitoring system disclosed in the JP-A Nos. 2003-282546 and 2005-236199.
The plasma monitoring system includes a plasma processing apparatus 10. The plasma processing apparatus 10 is an apparatus applying a radio frequency (hereinafter, “RF”) bias to a plasma chamber 11 set in a vacuum to generate plasma 12 within the plasma chamber 11, and performing such processings as etching and film formation on a wafer 20 that is a monitoring target workpiece disposed on a stage 13. A voltmeter 15 for self-alignment bias measurement is connected to the stage 13 via a coil 14 for alternating current (hereinafter, “AC”) voltage component elimination. A sensor 21 or the like for plasma process detection is bonded onto the wafer 20.
If a plasma process is to be monitored, then the plasma 12 is generated in the plasma chamber 11 by application of the RF bias to the plasma chamber 11, and the plasma process (e.g., plasma etching) is performed on the wafer 20. At this time, by monitoring a voltage detected by the sensor 21, a plasma etching end point may be detected and the wafer 20 may be worked with high accuracy.
It is generally known that energy of ions generated from the plasma 12 during the plasma etching influences a shape and a size of a pattern of the wafer 20 and electrically damages the wafer 20. Due to this, it is important to observe energy of ions incident on the wafer 20 from the plasma 12 and an ion energy distribution. However, since the ion incident energy if ions may not be directly measured, a self-alignment bias is monitored and set as an indirect index. Normally, the voltmeter 15 disposed below the stage 13 within the plasma chamber 11 measures an average value of the self-alignment bias. Since the self-alignment bias is an AC voltage, the coil 14 eliminates RF component in the AC voltage so that the voltmeter 15 may measure only a constant direct-current (hereinafter, “DC”) voltage.
FIG. 8 is a schematic diagram explaining the self-alignment bias. As shown in a state 1, when the wafer 20 is exposed to the plasma 12, the plasma 12 is in a state in which electrons e and positive ions h are slightly separated. Both the electrons e and the positive ions h move to be charged onto the wafer 20. However, at this time, the electrons e more faster and a large quantity of electrons e are charged onto the wafer 20 (and onto the stage 13 if the stage 13 is present under the wafer 20) since the electrons e are far lighter than the positive ions h. Due to this, as shown in a state 2, a potential of the wafer 20 turns negative by the charging of the electrons e on the wafer 20.
As shown in a state 3, the positive ions h which are oppositely charged to electrons e, and which move faster than electrons, arrive at the wafer 20. However, the amount of the positive ions h is not so large as to cancel the electrons e previously charged at the wafer 20. Due to this, ultimately both the negative electrons e and the positive ions h from the plasma 12 arrive at the wafer 20 and are charged thereat. However, since a charge amount of the initial negative electrons e (in the state 1) is larger, the potential of the wafer 20 is negative in a stable state. This negative potential is referred to as self-alignment bias.
Nevertheless, the conventional plasma monitoring methods and plasma monitoring systems have a first problem (1) and a second problem (2) as follows.
(1) First Problem
In a working process of forming a large scale integrated circuit (hereinafter, “LSI”) on the wafer 20, plural contact holes is formed, for example, by plasma etching. However, both a potential of a surface of the wafer 20 and that of a bottom of each contact hole may not be monitored in the conventional technique. Due to this, charge offset caused by trapping of charges (charge-up) may not be measured. If an aspect ratio (a ratio of a depth of each contact hole to a diameter thereof) is high, it is difficult for the electrons e to arrive at bottoms of the contact holes (electron blocking effect). As a result, the electrons e are insufficiently supplied to the bottoms of the contact holes, thereby making the bottoms of the contact holes positively charged up as compared with a surface of a contact hole pattern. These respects provoke such problems as dielectric breakdown of transistors, reduction in etch rate, and insufficient progress of etching. The charge-up problem is serious since the diameter of each contact hole in and after the advanced 65-nanometer (nm) generation is 0.1 micrometer (μm) and the aspect ratio of the contact hole is as high as about 10.
Generally, a recording memory transistor (Non-Volatile Memory Transistors (hereinafter “NVM”)) or a wafer (blank wafer), on which no circuit pattern is formed, is employed to monitor a charge-up amount. However, even with use of the NVM or the blank wafer, neither the measurement of a charge-up amount on an actual pattern nor that of a charge-up amount at real time may not be advantageously made. A problem related to the NVM (hereinafter, “(a) NVM-related Problem”) and a problem related to the blank wafer (hereinafter, “(b) blank wafer-related Problem”) will be described in detail.
(a) NVM-Related Problem
In case of the NVM, an antenna (a conductor) on the surface of a wafer 20 exposed to the plasma 12 is connected to a gate electrode of the NVM buried in the wafer 20. A transistor characteristics (easiness of current flow between a source electrode and a drain electrode) of the NVM changes according to a magnitude of a potential applied to the gate electrode of the NVM. Due to this, if charge-up occurs on the NVM charge-up monitoring wafer 20, charges are trapped into the antenna and a potential of the antenna changes. Since the antenna is connected to the gate electrode of the NVM, the characteristic of the NVM changes according to a potential of the antenna. Namely, an amount of a change in the transistor characteristics may be recognized from a magnitude of a charge-up amount (a potential change width). Therefore, if the charge-up occurs on the NVM charge-up monitoring wafer 20, then charges are trapped into the antenna and the antenna potential changes. Since the antenna is connected to the gate electrode of each NVM, the NVM characteristic changes according to the magnitude of the antenna potential. Namely, the magnitude of the charge-up amount (potential change width) may be confirmed from the change amount of the transistor characteristics. Accordingly, in case of the NVM, the sensor wafer 20 that is the monitoring target workpiece is temporarily exposed to the plasma 12 to change the NVM characteristic, the sensor wafer is taken out from the plasma 12, and how much the NVM characteristic changes (a change amount of the easiness for current flow across the NVM) before and after the exposure to the plasma 12 is measured using a measuring instrument.
Therefore, if the charge-up occurs in the atmosphere of the plasma 12, the charge-up (e.g., antenna potential) may not be observed at real time. Further, since the antenna (conductor) is flat and the flat antenna (conductor) receives (picks up) the charge-up, the charge-up that occurs in a pattern of an actual LSI product such as contact hole may not be measured.
(b) Blank Wafer-Related Problem
The blank wafer means a wafer configured so that only a silicon oxide film or a silicon nitride film is formed simply on one surface of a silicon substrate. If the wafer 20 having such an insulating film formed on the silicon substrate is exposed to the plasma 12, a surface of the insulating film is charged up. Next, when the wafer 20 is taken out from the plasma chamber 11, charges trapped onto the insulating film remain (as a charge-up residue). This charge-up residue is measured using a noncontact potential measuring instrument to thereby measure a charge-up degree. As can be seen, if the blank wafer is used, the measurement is made after the sensor wafer 20 is taken out from the atmosphere of the plasma 12 and not made when charge-up actually occurs in the atmosphere of the plasma 12. Therefore, the charge-up may not be measured at real time. Besides, since the insulating film is a plain film without a pattern on the silicon substrate, the charge-up may not be measured in an actual pattern including contact holes.
(2) Second Problem
Since the energy of ions incident on the wafer 20 from the plasma 12 may not be directly measured, the self-alignment bias is monitored and used as an indirect index. Normally, the average value of the self-alignment bias is measured by the voltmeter 15 disposed below the stage 13. Due to this, an in-plane distribution of the self-alignment bias may not be measured. This second problem will be described in detail.
As shown in FIG. 7, normally, the stage 13 is a conductive electrode. If the self-alignment bias is generated in the atmosphere of the plasma 2, the self-alignment bias is applied to portions (such as an outer circumference) of the stage 13 to which portions the plasma 12 is exposed. The voltmeter 15 is disposed below and connected to the stage 13, and measures the self-alignment bias. Due to this, the self-alignment bias is measured while using an entire area of the portions (e.g., the outer circumference) of the stage 13 to which portions the plasma is exposed as an antenna. As a result, how the self-alignment bias differs among plural points on the wafer 20 (on the stage 13) and the like may not be measured. In FIG. 7, the average value of the self-alignment bias with the outer circumference of the stage 13 set as an antenna (i.e., the average value of each self-alignment biases that possibly slightly differ among various points on the outer circumference of the stage 13) is measured.